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Ch. 7 - Transcendental Functions
Hass - Thomas' Calculus 15th Edition
Hass15th EditionThomas' CalculusISBN: 9780137616077Not the one you use?Change textbook
All textbooksHass 15th EditionCh. 7 - Transcendental FunctionsProblem 7.5.66
Chapter 7, Problem 7.5.66

Indeterminate Powers and Products
Find the limits in Exercises 53–68.
66. lim (x → 0⁺) x (ln x)²

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Identify the form of the limit as \(x \to 0^+\) for the expression \(x (\ln x)^2\). Notice that as \(x\) approaches \(0\) from the right, \(x\) approaches \(0\) and \(\ln x\) approaches \(-\infty\), so \((\ln x)^2\) approaches \(+\infty\). This creates an indeterminate form of type \(0 \cdot \infty\).
Rewrite the expression to transform the product into a quotient, which is often easier to analyze with L'Hôpital's Rule. For example, write \(x (\ln x)^2\) as \(\frac{(\ln x)^2}{\frac{1}{x}}\).
Check the new limit form as \(x \to 0^+\). Since \(\ln x \to -\infty\), \((\ln x)^2 \to +\infty\), and \(\frac{1}{x} \to +\infty\), the limit becomes an indeterminate form \(\frac{\infty}{\infty}\), suitable for applying L'Hôpital's Rule.
Apply L'Hôpital's Rule by differentiating the numerator and denominator separately with respect to \(x\). The derivative of the numerator \((\ln x)^2\) is \(2 \ln x \cdot \frac{1}{x}\), and the derivative of the denominator \(\frac{1}{x}\) is \(-\frac{1}{x^2}\).
Simplify the resulting expression after differentiation and analyze the new limit as \(x \to 0^+\). This process will help determine the behavior of the original limit.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Limits Involving Indeterminate Forms

Limits that result in expressions like 0·∞ or ∞·0 are called indeterminate forms. These require algebraic manipulation or special techniques to evaluate, as direct substitution does not yield a clear answer.
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Behavior of the Natural Logarithm Near Zero

As x approaches 0 from the positive side, ln(x) approaches negative infinity. Understanding this behavior helps analyze expressions like x(ln x)², where the logarithm term grows without bound in magnitude.
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Derivative of the Natural Logarithmic Function

Techniques for Evaluating Limits (e.g., Substitution and L'Hôpital's Rule)

To evaluate limits involving indeterminate forms, techniques such as rewriting the expression, substitution, or applying L'Hôpital's Rule can be used. These methods transform the limit into a determinate form that can be evaluated directly.
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Finding Limits by Direct Substitution
Related Practice
Textbook Question

In Exercises 25–36, find the derivative of y with respect to the appropriate variable.

27. y = (1 - θ)tanh⁻¹(θ)

Textbook Question

Each of Exercises 25–36 gives a formula for a function y=f(x). In each case, find f^(-1)(x) and identify the domain and range of f^(-1). As a check, show that f(f^(-1)(x))=f^(-1)(f(x))=x.


f(x) = 1/x², x > 0

Textbook Question

Evaluate the integrals in Exercises 39–56.

55. ∫dx/(2√x + 2x)

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Textbook Question

Evaluate the integrals in Exercises 33–54.

∫₀^(π/4) (1 + e^(tan θ)) sec²θ dθ

Textbook Question

Solve the initial value problems in Exercises 87 and 88.

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Textbook Question

Evaluate the integrals in Exercises 53–76.

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